1. Field of the Invention
The present invention relates to sterilization or sanitation of a surface or volume, especially a room or building, using a sterilizing mist. In particular, the invention pertains to using a sterilizing mist delivered at ambient pressure and having droplets generally less than 10 micron, preferably less than a micron and of sufficient throughput and reactivity to be effective in the elimination of bacteria using various concentrations of a biocide such as hydrogen peroxide solution. The methodology provides for extraction and delivery of a stable sterilizing mist dispersed in gas-like manner for treatment of items, space or surfaces in a volume.
2. Description of the Prior Art
As demonstrated by the ongoing efforts beginning in November 2001 to decontaminate federal buildings of anthrax, a continuing need exists for improved methods for sanitizing items and areas like those found within buildings and dwellings. Anthrax poses a specific danger in that these instances in the year 2001 involved anthrax spores that were delivered in sealed envelopes and that leached or became airborne and contaminated surfaces and spaces. Ultimately, chlorine dioxide gas had to be used to decontaminate the federal buildings involved. This prior method of using chlorine dioxide gas came at great expense because of the toxic hazards of chlorine dioxide gas to humans and because of the damage chlorine dioxide gas causes to documents and other items within the area treated. An equally effective, yet safer and less obtrusive, alternative would have been preferred if available.
Other general methods of sterilization and bacteria eradication have usually included heating, autoclaving, retorting, and pasteurization. Chemical methods used in the past have included gas or vapor sterilization using ethylene oxide or hydrogen peroxide vapor, and the low temperature gas plasma, STERRAD. Further, chemicals have been commonly used in the form of liquids, gels, and stored pressure aerosols for application to surface areas. Various forms of gamma, electron-beam, x-ray, and ultraviolet radiation have been used in sterilization, and filter sterilization is sometimes used.
Because of it strong oxidizing power, hydrogen peroxide acts as a sterilizing agent, but has not been used successfully for sterilizing surfaces and areas like those within buildings and dwellings. Hydrogen peroxide in gasified vapor phase is known to be unstable and decomposes to water and oxygen. Otherwise, sterilizing an area by providing hydrogen peroxide in gasified vapor phase could have been considered. However, because of its instability in the gas phase, vaporized hydrogen peroxide is inefficient for use in sterilization. With hot air flow assisted gasification/vaporization, considerable decomposition of hydrogen peroxide into H2O (water) and oxygen occurs very rapidly. The pre-decomposition of the hydrogen peroxide before reaching the bacterial site or intended surface is of great disadvantage because it decreases the reactivity and effectiveness of the agent in achieving the desired killing power because less reactive oxygen will contact the bacterial site.
Further, known methods that are effective in using hydrogen peroxide as a sterilizing agent have not been conducive to use for sterilizing articles using a hydrogen peroxide agent alone or in sterilizing items and surfaces in larger areas such as rooms in buildings and dwellings. For instance, recent gas plasma approaches using hydrogen peroxide are limited to fixed volume applications such as to sterilize surgery equipment or items in specially designed cabinets of only several liters in volume. Other approaches have not permitted a sufficient exposure to the hydrogen peroxide agent to be effective for sterilization without it being coupled with a secondary sterilizing mechanism.
U.S. Pat. No. 4,680,163 to Blidschun et al. teaches a process and apparatus for sterilizing open-topped containers in which a sterilizing agent such as hydrogen peroxide is ultrasonically atomized into electrically charged droplets to be deposited on the inner surfaces of a container. As described by Blidschun, the hydrogen peroxide droplets produced using ultrasonic atomization are in the range of 2-4 μm. Blidschun teaches that the use of an electrostatic field is necessary to convey the droplets to the surface of the container to be sterilized in a shorter time. The use of electrostatic charging provides sufficient rate of deposition of droplets to sterilize the container surface before the atomic oxygen formed by H2O2 decomposition recombines to form an oxygen molecule. However, using electrostatic charge to speed deposition of droplets is not practical or viable for sterilizing unpredictable and/or physically complex surfaces or articles and areas of larger volume such as encountered when entire rooms or buildings must be sterilized. Thus, Blidschun does not teach a methodology utilizing hydrogen peroxide as a sterilizing agent that would be applicable to the need for an improved method for sanitizing a variety of articles and surfaces in areas in large volumes such as buildings and dwellings, where items or surfaces cannot be readily sterilized using a wet aseptic process or charged electrostatically.
U.S. Pat. No. 4,366,125 to Kodera et al. teaches the use of ultrasonically atomized hydrogen peroxide mist of about 10 μm in diameter as a sterilizing agent that is very weak but is effective when used in a combination of steps including ultraviolet radiation. Following the teaching of Kodera would suggest that ultrasonically atomized hydrogen peroxide in mist form could not be effective in decontaminating articles and surfaces in a room or building. As in Blidschun discussed above, Kodera suggests that H2O2 mist on scale of several microns or more as described would decompose and the atomic oxygen recombine to form an oxygen molecule before effective sterilization by the hydrogen peroxide mist could take place. Thus, producing a stable mist effective for sterilization has been very difficult and elusive in the prior art.
Moreover, it has been found, as suggested by Blidschun and Kodera, that the ultrasonic method of producing a sterilizing mist has serious limitations. In particular, scaling a sterilizing mist to larger throughputs makes accomplishing efficient aerosolization to make a stable mist very difficult. The mist droplets revert and coalesce to form water, thereby reducing throughput of sterilizing mist product drastically and making the mist insufficient for sterilizing. Likewise, prior art does not discuss or provide methodology that would achieve throughputs beyond about 50 ml/min of mist. Heretofore, methods to extract and deliver such levels of mist throughput have been unavailable.
In summary, a method of using hydrogen peroxide in a stable gas-like phase that would be effective for sterilizing an area has been unavailable and infeasible. Known methods of using hydrogen peroxide in a mist form did not provide sufficient throughput, production quality of mist, and mass flow to merit consideration for use in a gas-like mist for volume application in sanitizing articles and surfaces in areas such as rooms, buildings, or air ducts. Therefore, as illustrated by the discussion of prior art, hydrogen peroxide mist previously has been used only in wet aseptic processes rather than in volume applications utilizing hydrogen peroxide in any gas or gas-like form.
Thus, a method is needed of producing a mist using ultrasonic atomization, in-situ aerosolization, and extraction processes to scale, transport and disperse the sterilizing mist to a volume whereby throughput, production quality and mass flow of the mist are controlled and suitable for volume fumigation and sterilization.